Unfortunately, the CDC reports that the disparity in mortality rates is not well understood:

A large body of literature has attempted to explain the much higher perinatal and infant mortality rates for black women. An important intermediate variable in this discussion is the much higher rate of preterm delivery for non-Hispanic black mothers, compared with non-Hispanic white and Hispanic mothers, but the reasons for this disparity are not well understood. Factors frequently mentioned as contributing to the black-white perinatal mortality gap are racial differences in maternal preconceptional health, infection, income, access to quality health care, stress and racism, and cultural factors; however much of the black-white disparity in perinatal mortality remains unexplained.